1. The problem statement, all variables and given/known data Pressure drops in welders tank of oxygen gas from p1=150atm to p2=120atm . How much of the gas will be used ? 2. Relevant equations 3. The attempt at a solution In my mind the simplest way of looking at this problem is to consider the process isothermal. But then the volume of the gas increases when pressure drops (Ideal gas law). This contradicts the question. Since I do not know the temperatures before and after, I dont know what to do. Im looking for different view at this problem, some help?